1. Field of the Invention
The present invention relates to an optical waveguide second harmonic generating element (SHG element), and more particularly to a second harmonic generating element suitable for use in a compact laser light source of a short wavelength to be used in fields of optical information processing, optical measurement and medical treatment, and a method of making the same.
2. Description of the Related Art
An oscillation wavelength of a currently practical semiconductor laser is in a range of infrared to red, and a semiconductor laser which oscillates a light of a shorter wavelength, for example, a green or blue wavelength has not yet been put into practice. Accordingly, it has been proposed to generate a second harmonic from an output light of a semiconductor laser, which oscillates an infrared light, thereby to produce a laser beam of a short wavelength.
In order to efficiently convert a wavelength of a light to a second harmonic, it is necessary to meet a phase matching condition so that phases of a number of second harmonics generated at various points in the SHG element match each other. To this end, methods of using angular phase matching, temperature phase matching or a waveguide have been known. Recently, a quasi-phase matching method using a periodic structure shown, for example, in Phys. Rev. Vol. 127, Sep. 15, 1962, J. A. Armstrong et al. pages 1918-1939 is attracting an attention in the field. In this method, a polarization direction in a crystal is periodically reversed to compensate for phase mismatching between a fundamental wave and a harmonic.
The polarization reversal means the reversal of the polarization direction of a single crystalline dielectric material polarized in a predetermined direction. When applied to an SHG element, it is called a polarization reversal type SHG element.
Various methods have been practiced to attain the polarization reversal. For example, in a lithium niobate (LiNbO.sub.3) or (LN) single-crystal, thermal diffusion of a Ti metal is used. In a lithium tantalate (LT) single-crystal, a method of rapid heating after proton exchange is used.
FIG. 6 shows an SHG element in which polarization reversal areas and polarization non-reversal areas are formed alternately in a comb shape in a substrate of LN or LT crystal, and a channel waveguide 20 is formed orthogonally thereto.
In generating a second harmonic, the following relationship exists between power of a fundamental wave (frequency .omega.) and a power of a second harmonic (frequency 2.omega.) EQU P(2.omega.).infin.{S(n,n,m)}.sup.2 .times.{P(.omega.)}.sup.2 /W(1)
where P(.omega.) and P(2.omega.) are powers of the fundamental wave and the second harmonic, respectively, W is a width of the waveguide, S(n,n,m) is a spatial coupling constant representing overlap of electromagnetic field distributions of the fundamental wave and the second harmonic, and n and m are orders of mode of the second harmonic and the fundamental wave, respectively. EQU {S(n,n,m)}=.intg.f.sup.2 (n,.omega.).multidot.f(m,2.omega.)dS(2)
where f(n,.omega.) and f(m, 2.omega.) are electric field distributions of the fundamental wave and the harmonic, respectively, and S is a cross-section.
In an SHG element which uses the quasi-phase matching by using the lithium niobate (LiNbO.sub.3) single-crystal which is one of the proposed SHG element; as disclosed in Electron Lett. Vol. 25, No. 3, Feb. 2, 1989, pages 174-175, by E. J. Lim and M. M. Fejer, a refractive index of an optical waveguide is changed by the generated P(2.omega.) because the lithium niobate is weak to light damage so that P(2.omega.) does not increase in proportion to the increase of a term {P(.omega.)}.sup.2 in the formula (1) but it tends to saturate. Further, since the refractive index of the optical waveguide changes with time, it is difficult to stabilize P(2.omega.) over a long time periods.
In order to solve a drawback of the SHG element which uses lithium niobate, a structure shown in FIG. 5 has been proposed as an SHG element which uses KTP (KTi OPO.sub.4) which has been known to be stronger to the light damage by the order of two figures than lithium niobate (Appl. Phys. Lett. Vol. 57, No. 20, Nov. 12, 1990, pages 2074-2076, by C. J. Van der Poel, J. D. Bierlein and J. B. Brown). In FIG. 5, numeral 51 denotes a substrate sliced at a z-plane of a KTP single-crystal, and numeral 52 denotes a Rb ion exchange waveguide made by exchanging a part of K ions by Rb ions. When Ba ions are added during the manufacture of the Rb ion exchanqe waveguide, the direction of spontaneous polarization of the waveguide portion is reversed relative to the bulk or substrate so that the SHG is attained by the quasi-phase matching. However, since the optical waveguide is discontinuous in the structure, a light passes through a bulk crystal portion in which no waveguide is formed when the light propagates from a waveguide portion to a next waveguide portion. Since a light confinement effect disappears in the bulk crystal portion, both the incident beam and the generated second harmonic beam spread. Namely, P(.omega.) in the formula (1) attenuates as the light travels. As a result, a power of P(2.omega.) as calculated is not attained.
The optical waveguide which uses KTP is also disclosed in Appl. Phys. Lett. Vol. 50, No. 18, May 4, 1987, pages 1216-1218, by J. D. Bierlein et al.